Filamentous fusion phage display the amino acids coded by a cloned DNA insertion the surface of an infectious virus particle. Phage bearing a particular foreign determinant can be affinity-purified with antibody, making it easy to purify clones that are as rare as 1 in 100 million in the original mixture. Methods will be devised for constructing fusion-phage "libraries" containing 100 million different foreign DNa inserts, so that the full potential of the technology can be exploited. The concept of an epitope library exemplifies the uses to which fusion phage might be put. The foreign DNA inserts in this library would be synthetic DNa with random sequence. Billions of short amino acid sequences would be represented in a 100-million-clone library; it is likely, therefore, that it will contain short determinants ("epitopes") recognized by any anti-protein antibody. This idea will be tested with antibodies directed against myohemerythrin, a small protein whose antigenic structure has been intensively studied with synthetic peptides. The results should yield a wealth of new information about the specificity of anti-protein antibodies-- information that touches on the fundamental issue of how the immune system manages to specifically recognize a seemingly unlimited repertoire of different protein antigens. They should also indicate the feasibility of future applications of the epitope library. One possibility is that it could be used to determine the epitope recognized by an interesting antibody--one, say, that confers protective immunity to some disease. This information could then be used to design a synthetic vaccine or immunogen capable of eliciting similar antibodies in other individuals--all without having to clone and analyze the gene encoding the natural antigen.